It is well known in the plastic molding art to use pressurized fluid in conjunction with the injection molding of articles. The pressurized fluid is typically nitrogen gas which is introduced into the molten plastic.
Pressurized fluid serves many purposes. First, it allows the article so formed to have hollow interior portions which result in weight and material savings. Secondly, it minimizes the molded in stresses by eliminating high second stage injection pressure. This also reduces part weight in that the gas is used to fill out the part. Thirdly, the pressurized fluid applies outward pressure to force the plastic against the surfaces of the mold cavity while the article is setting up. This enhances surface quality by eliminating sink marks, especially in areas of the article having thicker plastic sections, such as structural ribs or bosses.
One problem associated with fluid-assisted injection molding is that some parts are of such a size and shape so as to reduce the benefits normally obtained from the use of fluid-assisted injection molding. For example, for some items it is near impossible for the gas introduced at the injection aperture to move to the farthest reaches of the mold cavity. Very complex channel configurations throughout the part would be required to put the gas in a local area where it is needed and to the farthest reaches of the mold.
U.K. Patent Application 2,139,548 discloses an injection molding process wherein pressurized fluid is injected into a stream of plastic material at one or more selected positions in the mold space.
U.S. Pat. No. 4,474,717 discloses a localized fluid-assisted injection molding method and system including a probe having a head with one or more fluid orifices. In one embodiment the probe is retractable.
West German Patent Document 2,106,546 discloses a method and apparatus for forming plastic shoe heels with compressed gas blown into the center of the plastic mass. The resulting hollow cavity is then filled with a synthetic foam.
Part requirements and tooling considerations (i.e. thin steel sections), however, may prevent the introduction of gas directly into the part.